title: Comparison of fatigue strength of C2 pedicle screws, C2 pars screws, and a hybrid construct in C1-C2 fixation.
abstract: METHODS
  A biomechanical study comparing the fatigue strength of different types of C2 fixation in a C1-C2 construct.
  
  
  OBJECTIVE
  To determine the pullout strength of a C2 pedicle screw and C2 pars screw after cyclical testing and differentiate differences in stiffness pre- and post-cyclical loading of 3 different C1-C2 fixations.
  
  
  BACKGROUND
  Some surgeons use a short C2 pars screw in a C1-C2 construct, because it is less technically demanding and/or when the vertebral artery is high riding. Difference in construct stiffness between use of bilateral C2 pedicle screws, bilateral C2 pars screws, or a hybrid construct is unknown.
  
  
  METHODS
  Biomechanical testing was performed on 15 specimens. A bicortical C1 lateral mass screw was used in combination with 1 of 3 methods of C2 fixation: (1) bilateral long C2 pedicle screws (LL), (2) bilateral 14-mm C2 pars screws (SS), and (3) unilateral long C2 pedicle screw with a contralateral 14-mm C2 pars screw (LS). Each construct was subject to 16,000 cycles to simulate the immediate postoperative period. Changes in motion in flexion-extension, lateral bending, and axial rotation were calculated. This was followed by pullout testing.
  
  
  RESULTS
  The ability to limit range of motion significantly decreased after cyclical testing in flexion-extension, lateral bending, and axial rotation for all 3 groups. After loading, the LL and LS groups had less percentage of increase in motion in flexion-extension and lateral bending than the SS group. Overall, the average pullout strength of a pedicle screw was 92% stronger than a pars screw.
  
  
  CONCLUSIONS
  C2 pedicle screws have twice the pullout strength of C2 pars screws after cyclical loading. In cases in which the anatomy limits placement of bilateral C2 pedicle screws, a construct using a unilateral C2 pedicle screw with a contralateral short pars screw is a viable option and compares favorably with a bilateral C2 pedicle screw construct.
  
  
  METHODS
  N/A.
authors.affiliation: *Mt. Tam Orthopedics and Spine Center, Marin General Hospital, Larkspur, CA †University of Virginia, Charlottesville, VA ‡Globus Medical Inc., Audubon, PA §UCSF Department of Neurosurgery, San Francisco, CA; and ¶Thomas Jefferson University, Rothman Institute, Philadelphia, PA.
authors.forename: Brian W
authors.initials: BW
authors.lastname: Su
authors.forename: Adam L
authors.initials: AL
authors.lastname: Shimer
authors.forename: Suresh
authors.initials: S
authors.lastname: Chinthakunta
authors.forename: Kanaan
authors.initials: K
authors.lastname: Salloum
authors.forename: Christopher P
authors.initials: CP
authors.lastname: Ames
authors.forename: Alexander R
authors.initials: AR
authors.lastname: Vaccaro
authors.forename: Brandon
authors.initials: B
authors.lastname: Bucklen
mesh_terms: D001268:Atlanto-Axial Joint / Q000601:surgery*; D001368:Axis, Cervical Vertebra / Q000601:surgery*; D001696:Biomechanical Phenomena; D001863:Bone Screws; D001270:Cervical Atlas / Q000601:surgery*; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000379:methods*